1. Technical Field
The disclosure relates to a three-dimensional data processing technique for two-dimensionally displaying a three-dimensional space using stereographic data pertaining to a solid body.
2. Description of the Related Art
Computational processing by a computer plays a central role in two-dimensionally displaying a three-dimensional space using stereographic data pertaining to a solid body, namely, in rendering three-dimensional graphics. Computers with a graphic processing function have achieved higher performance and become more integrated (on one chip), which has made the application of a three-dimensional graphics rendering function possible in various technical fields, such as a navigation device mounted in an automobile or the like, for example, thus realizing a three-dimensional map display.
According to a ray tracing method utilized for such rendering of three-dimensional graphics, a ray is traced from a viewpoint toward the direction of a solid body. If the ray and the object intersect, then the mode of light of the object at the intersection point is found, and a two-dimensional image is created by projecting this on an image screen. However, an intersection point calculation to find the intersection of the ray and the object takes an extremely long time and a faster method is needed. For example, excluding unnecessary solid bodies within a view volume, i.e., a three-dimensional space targeted for a two-dimensional display, from a rendering target in advance can reduce the rendering process (three-dimensional image drawing process) load. Such an exclusion process is called a culling process, and there is a method that utilizes a bounding volume which can further speed up the culling process.
A conventional bounding volume has a simple geometry such as a sphere (bounding sphere) or rectangular solid shape (bounding box), and encloses one or a plurality of solid bodies. In other words, the bounding volume is a simple geometric body capable of covering an entire solid body. A drawing process can be performed faster by determining whether the bounding volume and the ray intersect, and then sending only the solid body enclosed by the bounding volume where the bounding volume and the ray intersect to a final drawing process, thereby eliminating a computation to find intersections of the ray with unnecessary solid bodies. Various methods for setting such a bounding volume are known (see Japanese Patent Application Publication No. JP-A-H06-168340, paragraphs 0001 to 0007, and Japanese Patent Application Publication No. JP-A-2003-271988, paragraphs 0002 to 0022, for examples).
In the above conventional method for setting the bounding volume, the bounding volume is set so as to enclose the solid body. Therefore the bounding volume has a size larger than the solid body in all directions. Accordingly, a forward bounding volume located in front of the viewpoint and represented with an excessive size hides a rearward bounding volume that is located therebehind, and the rearward bounding volume is thus culled. However, this creates a problem in that a solid body that should be visible in the three-dimensional image ultimately rendered cannot be seen (is not drawn). In particular, a bounding volume used in a map navigation system or the like in particular uses a bounding box parallel to reference axes and whose sides when cross-sectioned are parallel to a reference longitudinal axis and a reference lateral axes, which are commonly known as east longitude and north latitude coordinate axes. For this reason, the bounding box is considerably larger than the actual solid body (a polygon of the solid body), thus exacerbating the problem. When a bounding box parallel to reference axes is used, a visual axis extending from the viewpoint to the solid body becomes inclined with respect to the reference axes, and a region masked by the bounding box parallel to the reference axes becomes significantly larger than a region masked by the actual solid body, thus further exacerbating the above problem.
In light of the foregoing problem(s), there is a need to provide a three-dimensional data processing technique for generating a geometric object simplified into a bounding volume while also making a three-dimensional image ultimately rendered appear more natural, a three-dimensional image generating device using such a three-dimensional data processing technique, and a navigation device incorporating the three-dimensional image generating device.